Father's Day Poloniex Ethereum theft and questions ?

I had some questions I hope a Steemster could help me with.

Within the last month I finally took the plunge and bought some cryptocurrencies. I was talking to my Dad about them and he had never heard of them, but seemed really interested. So I had the idea for Father's Day to open a Poloniex account for him and give him a little Ethereum, Bitcoin and Litecoin to get started (just token amounts). That way he could play around or add to them if he wanted.

Wanting to surprise him I created a new email account and Poloniex account for him and moved the funds. I didn't do the two-factor authentication as I thought he could add it with his phone and I'd give him the account info on Father's Day.
The next day I logged into his account and his Ethereum had been transferred to someone's account (fortunately it was only $15!)
The Bitcoin and Litecoin was still there. So I changed passwords and added two-factor authentication.

My questions are:
How would someone get the wallet address so quickly and how did they know the password?
Do you think I have a virus on my computer?
Will the two-factor authentication protect the wallets or should I move the funds and close the Poloniex account?
Is this a common occurrence with cryptocurrencies?

Hopefully this may save another cryptocurrency newbie from losing funds. For me I'm lucky it wasn't a more expensive lesson!

Thanks in advance for your help!

Silver-Saver